Silicon dioxide, that silicon-oxygen combination known as quartz, makes up about 20 percent of the total crust of the earth. (With other oxygen-silicon combinations, or silicates, the total may reach 75 percent.) Because of their regular vibration in response to electrical current, quartz crystals are used for frequency control in radios, TV, electronic watches, flight control, and every device that broadcasts an electronic wave. As gems, sometimes in the form of amethyst, tiger's eye, agate, or the related opal, quartz crystals are valued for their beauty. As sand, quartz is ubiquitous along our coasts and rivers, in lakes, in deserts, and in common soils. As sand too, it is used in sandblasting, in plaster and concrete, and in glass. How quartz is formed and how it is turned to all these uses makes for a diversified lesson, but Zim is up to keeping it coherent and clearly focused. In his hands, the subject seems a natural.